Fault current limiters (FCL) with the cores saturated by superconducting coils

ABSTRACT

A current limiting device ( 30, 40, 50, 60 ) comprising for each phase of an AC supply a closed magnetic core ( 31 ) of reduced volume and mass having first and second pairs of opposing limbs ( 32   a   , 32   b   ; 33   a   , 33   b ), and at least one AC coil ( 35   a   , 35   b ) enclosing opposing limbs ( 33   a   , 33   b ) of the magnetic core ( 31 ) and adapted for series connection with a load. A superconducting DC bias coil ( 34 ) encloses a limb) ( 32   a   , 32   b ) of the magnetic core ( 31 ) for saturating each of the opposing limbs ( 33   a   , 33   b ) in opposite directions by the bias coil ( 34 ). Under fault conditions, the AC flux in at least one limb counteracts the DC bias flux, bringing the limb out of saturation. Preferably, current is reduced in the DC bias coils thus bringing both opposing limbs of the core out of saturation.

FIELD OF THE INVENTION

This invention relates to superconducting current limiting devices forAC electric grid.

References

In the following description, reference will be made to the followingpublications:

-   [1] B. P. Raju, K. C. Parton, T. C. Bartram, “A current limiting    device using super-conducting d.c. bias: applications and    prospects,” IEEE Transactions On Power Apparatus & Systems, vol.    101, pp. 3173-3177, 1982.-   [2] J. X. Jin, S. X. Dou., C. Grantham, and D. Sutanto “Operating    principle of a high T-c superconducting saturable magnetic core    fault current limiter”. Physica C, 282, Part 4: p. 2643-2644, 1997.-   [3] J. X. Jin, S. X. Dou., C. Cook, C. Grantham, M. Apperley, and T.    Beals, “Magnetic saturable reactor type HTS fault current limiter    for electrical application”. Physica C, 2000. 341-348: p. 2629-2630.-   [4] V. Keilin, I. Kovalev, S. Kruglov, V. Stepanov, I. Shugaev, V.    Shcherbakov, I. Akimov, D. Rakov, and A. Shikov, “Model of HTS    three-phase saturated core fault current limiter”, IEEE Transactions    on Applied Superconductivity, vol. 10, pp. 836-839, 2000.-   [5] R. F. Giese, “Fault-current limiters—A second look,” Argonne    Nat. Lab., Argonne, USA Mar. 16, 1995.-   [6] WO 2004/068670 (Yosef Yeshurun et al.) published Dec. 8, 2004    “Fault current limiters (FCL) with the cores saturated by    superconducting coils.”

BACKGROUND OF THE INVENTION

Fault Current Limiters (FCL) are expected to be among the first and mostimportant power applications of High Temperature Superconductors (HTS).The advantages of HTS-FCL as compared to conventional current limitingdevices, used world-wide in national electricity circuits, are theirquick response and fast recovery, relatively low energy dissipation,tolerance to large fault currents and the possibility for virtuallyunlimited number of operations.

More particularly, the present invention relates to current limitingdevices based on a superconducting coil with saturated core. In knowndesigns, such a device comprises at least two coils with ferromagneticcores for each phase connected in series with a load. On the cores thereare superconducting bias coils connected to a DC power supply. At normalstate the bias coils saturate the cores, and the impedance of thecurrent limiter is very low. When a fault occurs, the current sharplyincreases and the cores are driven out of saturation at alternatehalf-cycles. As a result the impedance of the current limiter builds upand limits the current increase.

Two main designs of a saturated core reactor for limiting a faultcurrent in electric power system are proposed in U.S. Pat. No.3,219,918, incorporated herein by reference. One design includes two ACcoils placed on two outer legs of an E-core. Another design employs asingle AC coil that encompasses two legs belonging to different coresthat are saturated in opposite directions. In this patent DC coils madeof copper are envisaged.

In U.S. Pat. No. 3,671,810 incorporated herein by reference thisprinciple has been proposed for transient current limiting in electroniccircuits. U.S. Pat. No. 4,045,823 incorporated herein by reference to K.C. Parton et al describes a current limiting device for a poweralternating current system. The current limiter has for each phase apair of saturable reactors whose coils are wound in opposite directionsrelative to superconducting bias coils. U.S. Pat. No. 4,117,524incorporated herein by reference also to K. C. Parton et al. describes amodified form of current limiter having a screen of conductive materialsurrounding the bias winding to shield it against the alternatingmagnetic field. In this patent, one common bias coil is used for tworeactors. Raju et al. [1] realized their current limiting device with asuperconducting bias coil operating in a liquid helium bath anddemonstrate its efficiency. U.S. Pat. No. 4,257,080 (Bartram et al.)incorporated herein by reference describes a further improvement of thiscurrent limiting device by placing the common bias coil on the centrallimbs of three or six cores of a three-phase reactor. In the threementioned patents additional air-gapped cores are placed in the circuitof the bias coil. These cores are necessary for decreasing alternatingcurrent in DC circuit caused by transformer coupling between the ACcoils and bias coils.

Several laboratory scale models of saturable core current limiters havebeen realized with superconducting coils made of high-temperaturesuperconductors (HTS) [2, 3, 4]. These one-phase [2, 3] and three-phase[4] devices were built according the design proposed in theabove-mentioned US patents, the contents of all of which areincorporated herein by reference.

The current limiter with saturated core has decisive advantages ascompared with other superconducting current limiters:

-   -   its current limiting effect is not dependent on transition of        the superconducting element to normal state, i.e.        superconducting state is maintained all the time and no recovery        time is necessary to return to ready state after fault.        Moreover, there is no dissipation of energy associated with        transition of the super-conducting element to normal state;    -   the superconducting element is a coil made of standard        superconducting wire manufactured on an industrial scale;    -   the superconducting coil operates in DC mode and is exposed to        low AC magnetic fields.

Known designs of FCL with saturated cores have essential shortcomingsthat prevent development and realization of this type of FCL. Itsweakest points are the large weight and dimensions that are about twicethe weight and dimensions of a transformer of the same power [5]. Also,in known FCLs of this type the impedance of the AC coils does not reachits maximum possible value because the bias coils produce magnetic fluxin the cores that reduces the impedance of the AC coils. This feature isnecessary at normal conditions but has a negative effect at faultconditions. Furthermore, at fault conditions the alternating magneticfield of the AC coils affects the superconducting bias coil, decreasingits critical current. In known designs, a cryostat with bias coils isplaced in the window of the core thus increasing its size. The size ofthe magnetic core is defined mostly by its cross-section, which in turnis determined by the required voltage drop on the FCL during a fault.This voltage is proportional to the product of the cross-section of thecore with the number of turns in the AC coil. The number of turns islimited by allowable voltage drop on FCL at normal operation.

In the above-mentioned WO 2004/068670, we propose new designs thataddress these considerations. First, instead of closed magnetic cores,open cores (rods) are used. The weight of such core is less than of theclosed core. Second, an additional feedback coil is used to compensatethe magnetic flux of the bias coil at the fault regime thus increasingthe impedance of the FCL limiting the fault current. Use of theadditional feedback coil changes the properties of FCL in such a waythat both AC coils operate at fault regime during both half cycles. Itallows the cross-section of the core to be decreased because therequired voltage drop on the FCL is distributed between two coilsinstead of one at each half-cycle as occurs in previous designs.

However, the transformer coupling inherent in known configurationsinduces an AC voltage on the superconducting DC bias coil thussuperimposing an AC current component in the DC circuit. Moreover, thesame effect inheres also to the additional DC circuit of the feedbackcoil. In all state of art designs the bias coil has a number of turnsclose to the number of turns in the AC coil and thus the voltage on thebias coil has the same order of magnitude as on the AC coil, i.e. thevoltage of the grid at the time of fault.

JP2002118956A2 discloses a current limiter that includes a pair of firstand second magnetic cores facing each other, a closed magnetic circuitformed of permanent magnets jointed between the first and secondmagnetic cores, and a coil by winding a conductor around the first andsecond magnetic cores, through which saturation magnetic fluxesdeveloped by the permanent magnets flow. The first and second magneticcores, where the directions of saturation magnetic fluxes are oppositeto each other, are formed so that magnetization is reversed alternatelyby current passing through the coil at short-circuit for each halfperiod of the current.

It would therefore be desirable to provide an improved design of FCLhaving a superconducting bias coil wherein this drawback is addressedwithout compromising the advantages afforded by the configurationproposed in WO 2004/068670.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved design of FCL with saturated core that includes at least onesuperconducting DC bias coil placed on a single closed ferromagneticcore, which selves as open core for a single AC coil. Such design of acurrent limiter allows building the FCL with saturated core having asmall mass and dimensions and also reduces or eliminates the transformercoupling between the AC coil and the superconducting DC bias coil(s)thus decreasing AC voltage on the super-conducting DC bias coil(s).

A further object of the invention is to provide an improved currentlimiter with saturated core where the bias field is decreased oreliminated at the time of a fault by disconnecting the bias coils fromtheir power supply and connecting them in a voltage limiting circuitwith energy absorbing elements controlling maximal voltage on the coils.The disconnection is realized by a switching device, controlled by thevoltage drop on the AC coil, that also restores the DC coil circuitafter disconnecting the fault.

Yet another object of the invention is to provide switching of the DCcircuit that connects two bias coil segments in opposite directionsrelative to an initial connection for preventing a possible transformercoupling effect at the time of fault.

Additional objectives of the present invention are:

-   -   to reduce the alternating magnetic field on the superconducting        bias coils thus preventing a degradation of their critical        current;    -   a to reduce the number of Ampère-turns of the bias coils without        increasing the core size.

These objects are realized in accordance with a first aspect of theinvention by a current limiting device for an AC supply, said currentlimiting device comprising for each phase of the AC supply:

a magnetic circuit forming an open magnetic core for at least one ACcoil and forming a closed magnetic circuit for at least onesuperconducting DC bias coil that is adapted under non-fault conditionsto bias the magnetic core into saturation so that each of the opposinglimbs is saturated in opposite directions by the bias coil.

The magnetic circuit preferably comprises:

a closed magnetic core having a first pair of opposing limbs and asecond pair of opposing limbs,

at least one AC coil enclosing opposing limbs of the magnetic core andbeing adapted to be connected in series with a load, and

at least one superconducting DC bias coil enclosing at least one limb ofthe magnetic core and being adapted under non-fault conditions to biasthe magnetic core into saturation so that each of the opposing limbs issaturated in opposite directions by the bias coil.

Since the AC coil is commonly wound externally on both limbs of thecore, the AC coil sees an open core, opposing limbs of which aresubjected to AC flux in the same direction, which will alternate duringalternate half-cycles of the AC current. As against this, the DC biascoil is wound internally on the core in a way that forms a closedmagnetic circuit for the DC flux and affects the magnetic permeabilityof the complete core. Specifically, the DC bias coil ensures that thecore is magnetized whereby under non-fault conditions its magneticpermeability is low. Moreover, since the flux produced by the DC biascoil encircles the four limbs of the magnetic core in a fixed angulardirection (clockwise or anti-clockwise) determined by the direction ofthe DC current, it always acts in the same direction as the AC flux inone limb and in the opposite direction of the AC flux in the oppositelimb. The dimensions of the magnetic core and the number of turns of theAC coil are so designed that, even under maximum fault conditions, thecurrent in the AC coil does not bring the core into saturation.Therefore, even under maximum fault conditions, the AC flux adds to thesaturation produced by the DC bias coil in one limb; while in theopposite limb, the AC flux acts to bring the limb out of saturationproduced by the DC bias coil. The limb that remains in saturationexhibits low magnetic permeability, while the limb that is no longersaturated exhibits high magnetic permeability. What this means is that,in effect, under fault conditions some of the cross-sectional area ofthe magnetic core always contributes to high coil impedance and serves,thereby, to resist the fault.

Such an arrangement, whereby the AC coil is wound on an open magneticcore, while the DC bias coil is adapted under non-fault conditions tobias opposing limbs of the magnetic core into saturation in oppositedirections, has not been proposed previously and allows the effectivecross-sectional area of the magnetic core and/or the Ampère-tuns in theDC bias coil to be reduced.

In order to improve the efficiency of the device and bring the whole ofthe magnetic core out of saturation under fault conditions, the DCelectric circuit of bias coils is preferably supplied with a currentreduction unit that reduces the DC bias current during fault conditions.Better effectiveness is achieved where the current reduction unit isconstituted by a switching unit that disconnects the bias coils from theDC power supply at the time of fault and includes the bias coils andenergy absorbing elements that also limit the voltage on bias coils.

The switching enables the maximal voltage drop on the current limiter tobe increased as compared with an FCL without switching because both legsof the core are out of saturation and the effective cross-section of thecore is increased. An additional effect of using the switching unit (asa result of increasing the effective core permeability) is a strongreduction of the leakage AC field that has a negative influence on thesuperconducting bias coil. When the DC bias coils are energized, the DCflux always provides a positive offset to the AC flux in one of thelimbs and a negative offset to the AC flux in the opposite limb. Whenthe DC switches off, the magnetic picture becomes symmetric and alllimbs of the magnetic core are unsaturated, thereby contribute to highmagnetic impedance.

The switching unit allows the mass of the device to be reducedregardless of the type of core employed in the same way as describedabove in relation to the feedback coil.

In accordance with another aspect of the invention, there is provided amethod for reducing mass of a current limiting device for an AC supply,said current limiting device comprising for each phase of the AC supplya magnetic circuit that offers low impedance under non-fault conditionsand high impedance under fault conditions, said method comprising:

constructing the a magnetic circuit so as to form an open magnetic corefor at least one AC coil and forming a closed magnetic circuit for atleast one superconducting bias coil that is adapted under non-faultconditions to bias the magnetic core into saturation so that each of theopposing limbs is saturated in opposite directions by the bias coil;

whereby under fault conditions some of the cross-sectional area of themagnetic core always exhibits high permeability and serves, thereby, toresist the fault and allow the cross-sectional area of the at least oneAC coil and magnetic core to be reduced.

Preferably, said method further comprises:

reducing current in the at least one superconducting DC bias coil duringa fault condition thereby bringing the core out of saturation.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, some preferred embodiments will now be described, byway of non-limiting examples only, with reference to the accompanyingdrawings, in which:

FIG. 1 shows pictorially a prior art closed core configuration of asaturated core single phase FCL;

FIG. 2 is a schematic circuit diagram showing the prior art single phaseFCL of FIG. 1 in use;

FIG. 3 shows a magnetic circuit having a closed core for a single phaseFCL according to first exemplary embodiment of the invention;

FIG. 4 shows the form of a core for a single phase FCL according to asecond exemplary embodiment of the invention;

FIG. 5 shows a saturated core FCL according to a third exemplaryembodiment of the invention;

FIGS. 6 a and 6 b show a saturated core FCL according to a fourthexemplary embodiment of the invention;

FIG. 7 is a schematic circuit diagram showing a single phase FCLaccording to an exemplary embodiment of the invention with a switchingsystem for disconnecting power from the bias coils during a faultcondition; and

FIG. 8 is a schematic circuit diagram showing a single phase FCLaccording to an exemplary embodiment of the invention with a switchingsystem for disconnecting power from the bias coils and reconnecting themin mutually opposed relationship during a fault condition.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the following description various embodiments are described. To theextent that many features are common to different embodiments, identicalreference numerals will be employed to refer to components that arecommon to more than one figure.

In order more fully to appreciate the benefits of the invention, it willbe instructive first to consider a typical prior art single phase FCL.To this end, FIG. 1 shows pictorially a prior art saturated core singlephase FCL 10 having a magnetic circuit comprising a pair of closedmagnetic cores 11 a and 11 b each supporting a respective AC coil 12 aand 12 b. The cores further support a pair of DC superconducting biascoils 13 a and 13 b.

FIG. 2 shows schematically a circuit diagram of a system 20 showing thesingle phase FCL 10 in use. An AC supply 21, typically from the electricpower grid, is connected to a load 22 via a circuit breaker 23. Inseries with the load 22 are connected the two AC coils 12 a and 12 b ofthe FCL 10. The respective superconducting bias coils 13 a and 13 b areconnected to a DC power supply 24. At any moment the direction of thebias magnetic flux 25 a in one core coincides with the direction of themagnetic flux 26 a of the AC coil 12 a whereas the direction of the biasmagnetic flux 25 b in the other core is opposite to the direction of themagnetic flux 26 b of the AC coil 12 b. Under normal conditions, thebias coils 25 a and 25 b saturate the respective cores 11 a and 11 b.Under fault conditions, the AC coils 12 a and 12 b draw the respectivecores 11 a and 11 b out of saturation during opposite half cycles of theAC cycle, thereby causing their average inductance to increase, thuslimiting the current increase.

FIG. 3 shows a single phase FCL 30 according to a first embodiment ofthe invention having a magnetic circuit comprising a single closed core31 having opposing pairs of short legs 32 a and 32 b and long legs 33 aand 33 b as compared with the two closed cores of known designs as shownin FIG. 2. A single bias coil 34 is placed on one of the short legs 32 aof the closed core 31. In this embodiment only one AC coil 35 is usedthat encircles the two long legs 33 a and 33 b of the core in such a waythat the AC coil is disposed on the open magnetic core. An advantage ofsuch an arrangement of transverse AC and DC bias coils is thattransformer coupling of the coils is decreased, since the mutualinductance between the coils is ideally zero when the DC is off and theis AC fluxes in opposing limbs are equal and cancel each other at thecenters of transverse limbs.

FIG. 4 shows an FCL 40 according to a second exemplary embodimentsimilar to the first embodiment in which, instead of the single biascoil 34, two bias coils 34 a and 34 b are placed on the opposite shortlegs 32 a and 32 b of the core thus enabling better saturation of thecore with the same total number of Ampère-turns in the bias coils. Thisis achieved by splitting the original DC bias coil shown in FIG. 3 totwo coils while maintaining the Ampère-turns. This is done because areasin the core, which are remote from the coil, are less saturated thanareas close to the coil.

FIG. 5 shows an FCL 50 according to another exemplary embodiment havingan identical closed core 31 and a common AC coil 35 wound around thelong legs 33 a and 33 b of the core. Two DC bias coils 34 a and 34 b areplaced on the two long legs 33 a and 33 b of the core encompassed by theAC coil 35 thus enabling better saturation of the core with a smallernumber of Ampère-turns of the bias coils.

FIGS. 6 a and 6 b show an FCL 60 according to another exemplaryembodiment having a closed magnetic core 31 that is formed by foldingthe core 31 shown in FIG. 6 a and corresponding to that shown in FIG. 2,3 or 4 about a pair of lines 40 a-40 b and 41 a-41 b so as to form apair of spaced apart C-shaped cores 42 a, 42 b as shown in FIG. 6 b. TheC-shaped cores 42 a, 42 b face each other and the open ends ofrespective limbs of each core are magnetically coupled by legs 43 a and43 b so as to form a closed magnetic circuit. Two bias coils 34 a and 34b are wound on the legs 43 a and 43 b of the core. A first AC coil 35 aencloses the opposite limbs of the C-shaped core 42 a and a second ACcoil 35 b encloses the opposite limbs of the C-shaped core 42 b. Such aconfiguration enables better saturation of the core than in the firstembodiment shown in FIG. 3 and permits mounting both bias coils in asingle cryostat unlike the embodiment of FIG. 4 where the two bias coils34 a and 34 b are not capable of being inserted simultaneously into acommon cryostat.

All the above-described embodiments are characterized by an AC coil 35that encloses two limbs of the core magnetized to saturation in oppositedirections by the DC coils. The core is never saturated by the AC coilalone but only by the DC bias coils which magnetize the “AC limbs” inopposite directions during opposite half cycles of the AC supply. As aresult during a fault condition only one limb is driven out ofsaturation while the other limb is further drawn into deeper saturationif the DC bias coils continue to magnetize the core as is typically donein hitherto-proposed FCLs. However, if at the moment of fault, thecurrent in the DC bias coil or coils 34 is reduced as is done in theinvention, the maximal magnetic flux of the AC coil can be increasedwithout saturating the core, thus increasing the maximal allowablevoltage drop on the FCL. This effect is equivalent to decreasing thesize of the core because during a fault both limbs are driven out ofsaturation. As a result, the cross-sections of the AC coil and the corecan be reduced.

FIG. 7 is an exemplary schematic circuit diagram showing a system 70that includes the FCL 30 shown in FIG. 3, wherein the bias coil 34 isenergized by a DC supply 24 via a switching unit 71 that includes fasttransistor switch 72 and an energy absorbing element 73 limiting maximalvoltage in the electric circuit. The switching unit 71 thus serves toreduce current in the DC bias coil. Other elements are similar to thesystem 20 shown in FIG. 2 and have the same labeling. The system 70operates as follows. At any moment the magnetic flux in both limbs willbe directed to the left or to the right in the figure, since the AC coil35 is commonly wound on both limbs. Under normal conditions, the DC biascoil 34 saturates the core so that limbs 33 a and 33 b are saturated inopposite directions, and the AC coil 35 thus exhibits low impedance.Under fault conditions, the current through the AC coil increases and,for so long as the DC bias coil 34 remains effective, during alternatehalf cycles, the AC coil 35 de-saturates a respective one of the limbs33 a and 33 b. Therefore, the magnetic flux inside the AC coil 35 andits related inductance is defined by only one of the limbs 33 a and 33b, i.e. by half of the full core cross-section. However, if under faultconditions the switching unit 71 disconnects the DC power source 24 fromthe DC bias coil 34, its current falls down, thereby de-saturating thecomplete core including limbs 33 a and 33 b, and doubling the effectivecross-section of the core inside the AC coil 35 and increasing itsimpedance. This means that an equivalent current limiting effect can beachieved with such a topology having significantly reducedcross-sectional area of the AC coils and magnetic core compared withhitherto-proposed topologies.

The energy-absorbing element 72 is necessary to limit the voltage acrossthe coil 34 during the time of switching. During this transient timeregime the magnetic fluxes in limbs 33 a and 33 b are not equal and afast change of the magnetic flux in limbs 32 a and 32 b may induce analternating voltage/current on the bias coil(s) that might be harmfulfor the superconducting DC bias coils. The switching unit 71 not onlydisconnects the DC power source 24 from the DC bias coil 34 but alsoconnects the two DC bias coils 34 a, 34 b or two segments of one DC biascoil 34 in opposite directions thus minimizing the overall AC voltage inthe DC bias coils circuit and preventing AC current from flowingtherein. Two energy-absorbing elements 83 a, 83 b are necessary forlimiting the voltage on each DC bias coil or half coil. The voltage dropon the FCL triggers the switching circuit 71. When a fault occurs, thisvoltage changes abruptly by typically one order of magnitude allowingaccurate and reliable fault detection.

FIG. 8 is an exemplary schematic circuit diagram showing a system 80that includes the FCL 40 or 50 shown in FIGS. 4 and 5, respectively,having two DC coils 34 a and 34 b that are energized by a DC supply 24via a switching unit 81. The switching unit 81 includes first and secondfast transistor switches having normally closed contacts 82 a, 82 b andnormally open contacts 82 c, 82 d and corresponding first and secondenergy absorbing elements 83 a, 83 b that limit maximal voltage in theelectric circuit. Under fault conditions, the contacts 82 a and 82 bopen thereby disconnecting the DC supply 24 from the DC bias coils 34 aand 34 b; while, at the same time, the contacts 82 c and 82 d closethereby connecting respective DC bias coils 34 a and 34 b in anti-phaseso that the DC bias coils 34 a and 34 b are counter wound relative toeach other in a way that the possible induced voltage on both DC coilsand current therein are minimized. The energy absorbing elements 83 a,83 b limit the voltage on each of the bias coils.

It will be understood that modifications are possible to the exemplaryembodiments as described without departing from the scope of theinvention as claimed. Thus, in the exemplary embodiments, a switchingunit is used to disconnect the DC supply from the DC bias coils andthereby reduce the DC bias current to zero. Under these conditions, theAC fluxes in the opposing limbs of the magnetic core equal each other.However, the invention also contemplates reducing the DC bias current toless than zero. This will still work as at least half of the core'scross-section always is driven out of saturation by the AC coil current.Any reduction in the DC bias current adds to the effective cross-sectionparticipating in the limiting effect. Current reduction may be achievedusing feedback, for example, as taught in WO 2004/068670 or using anyother suitable method.

It will also be appreciated that the invention embraces any magneticcircuit forming an open magnetic core for at least one AC coil andforming a closed magnetic circuit for at least one superconducting biascoil that is adapted under non-fault conditions to bias the magneticcore into saturation so that each of the opposing limbs is saturated inopposite directions by the bias coil. Such a magnetic circuit hasutility for a current limiting device independent of the switching unit,even though without reducing the DC bias current the efficiency would belower. The term current reduction unit as used in the description andappended claims embraces any circuit for reducing DC bias current,whether the DC bias current remains non-zero or is disconnectedaltogether.

1. A current limiting device for an AC supply, said current limitingdevice comprising for each phase of the AC supply: a magnetic circuitforming an open magnetic core for at least one AC coil enclosingopposing limbs of the magnetic core and forming a closed magneticcircuit for at least one superconducting DC bias coil that is adaptedunder non-fault conditions to bias the magnetic core into saturation sothat each of the opposing limbs is saturated in opposite directions bythe bias coil.
 2. The current limiting device according to claim 1,wherein the magnetic circuit includes: a closed magnetic core having afirst pair of opposing limbs and a second pair of opposing limbs, atleast one AC coil enclosing opposing limbs of the magnetic core andbeing adapted to be connected in series with a load, and at least onesuperconducting DC bias coil enclosing at least one limb of the magneticcore and being adapted under non-fault conditions to bias the magneticcore into saturation so that each of the opposing limbs is saturated inopposite directions by the bias coil.
 3. The current limiting deviceaccording to claim 2, including: a single superconducting DC bias coilone limb of the first pair of opposing limbs, and a single AC coilenclosing the second pair of opposing limbs.
 4. The current limitingdevice according to claim 2, including: a pair of superconducting DCbias coils each enclosing a respective limb of the first pair ofopposing limbs, and a single AC coil enclosing the second pair ofopposing limbs.
 5. The current limiting device according to claim 2,including: a pair of superconducting DC bias coils each enclosing arespective limb of the second pair of opposing limbs, and a single ACcoil enclosing the second pair of opposing limbs.
 6. The currentlimiting device according to claim 2, wherein the magnetic coreincludes: first and second spaced apart C-shaped cores each having limbswhose respective open ends are magnetically coupled by respective legs,a pair of DC bias coils each enclosing a respective one of the legs ofthe core, a first AC coil enclosing opposite limbs of the first C-shapedcore, and a second AC coil enclosing opposite limbs of the secondC-shaped core.
 7. The current limiting device according to claim 1,further including a current reduction unit for reducing current in theat least one superconducting DC bias coil during a fault condition. 8.The current limiting device according to claim 7, wherein the currentreduction unit is adapted to disconnect the at least one superconductingDC bias coil from the power supply during a fault condition.
 9. Thecurrent limiting device according claim 7, wherein a respective energyabsorbing element is connected across the at least one superconductingDC bias coil.
 10. The current limiting device according to claim 7,wherein the current reduction unit is controlled by the voltage drop onthe at least one AC coil so as to reduce current in the bias coilsduring a fault condition and restore current in the bias coils after thedisconnection or termination of the fault.
 11. A method for reducingmass of a current limiting device for an AC supply, said currentlimiting device comprising for each phase of the AC supply a magneticcircuit that offers low impedance under non-fault conditions and highimpedance under fault conditions, said method comprising: constructingthe magnetic circuit so as to form an open magnetic core for at leastone AC coil and forming a closed magnetic circuit for at least onesuperconducting DC bias coil that is adapted under non-fault conditionsto bias the magnetic core into saturation so that each of the opposinglimbs is saturated in opposite directions by the bias coil; wherebyunder fault conditions some of the cross-sectional area of the magneticcore always exhibits high permeability and serves, thereby, to resistthe fault and allow the cross-sectional area of the at least one AC coiland magnetic core to be reduced.
 12. The method according to claim 11,further comprising: reducing current in the at least one superconductingDC bias coil during a fault condition thereby bringing the at least oneAC coil out of saturation and allowing a cross-sectional area of the ACcoils and magnetic core to be reduced.
 13. The method according to claim11, including: disconnecting the at least one superconducting DC biascoil from the power supply during a fault condition.
 14. The methodaccording to claim 12, wherein the magnetic circuit includes a pair ofsuperconducting DC bias coils and there is further included: connectingthe superconducting DC bias coils in anti-phase so as to minimizepossible induced voltage across and current through the superconductingDC bias coils.
 15. The method according to claim 11, further includingconnecting the at least one DC superconducting bias coil to a respectiveenergy absorbing element.